(1) Field of the Invention
The present invention relates to a failure detecting system for a communication system, and more particularly, to a failure detecting system for a user network connected to broad band ISDN in which data is transferred by using ATM cells, and which is connected to a customer station through a network terminator and a bus conformable to a DQDB protocol.
(2) Description of the Related Art
Broad-band ISDN (Integrated Services Digital Network) has been developed to allow practical broad-band communication such as video communication. The broad-band ISDN is currently realized by using an ATM (Asynchronous Transfer Mode). In the ATM, data is transferred by dividing it into fixed-length short packets called cells. The cells are exchanged and transferred by an ATM exchange or a node.
In the ATM, a circuit or, in other words, a line between a sender and a receiver, is not always provided, but a virtual circuit or virtual line is provided through which an essential ATM cell is transmitted. This allows a plurality of virtual circuits to share a single transmission line. To handle this line multiplication, a multiplication method called a statistical multiplication is employed to manage the statistical multiplication in such a way that the total data multiplied on a single transmission line does not exceed the capacity of the transmission line. Statistical multiplication ensures the efficient use of available circuits. Techniques have been developed to treat congestion when it occurs. For subscribers connected to a broad-band ISDN, a plurality of data receiving/transmitting devices called terminal equipments or terminal adapters in a customer station are connected through a bus. A DQDB (Distributed Queue Dual Bus) is considered to be favorable for such a bus (a user network). The present invention relates to a failure detecting system in an ISDN in which the communication between a local switch and each of the terminal equipments or terminal adapters is carried out in accordance with the DQDB protocol.
In the broad-band ISDN communication employing the DQDB protocol in the user network side, congestion may occur in an apparatus on the network side. Such congestion may occur in an ATM exchange, a local switch, or any processing device for processing ATM cells. To relieve this congestion on the network side, it can be considered to restrict the ability of a subscriber to transmit an ATM cell to the apparatus, in which the congestion has occurred.
In a conventional congestion relieving system, a network terminator inserts busy bits equal to "1" into busy bit areas of cells in accordance with a required rate of restriction of cells to be transmitted. This conventional system is disclosed in U.S. Pat. No. 5,231,630 and U.S. Ser. No. 831,794, filed on Feb. 5, 1992, both the U.S. Patent and the presently-pending U.S. patent application are commonly assigned with the present application. The cells having the busy bits equal to "1" are deemed as those being used by the terminal equipments or the terminal adapters. Therefore, no terminal equipment or terminal adapters insert data into cells with the busy bits set to "1". Accordingly, the cells with the busy bits="1" inserted at the network terminator have no set destination so that, when these cells are received by an apparatus such as an ATM exchange, they are deemed invalid and discarded. As a result, the number of cells to be transmitted to an apparatus, such as an ATM exchange, is decreased and the congestion is relieved. In a conventional system described above, however, a failure in transmission of each cell to a bus has not been considered.